During a blackout, On-Line or Line-interactive UPS systems both perform well, but On-Line UPS systems provide a smoother transfer between utility and battery backup power. Because its batteries are integrated into its normal operation, On-Line UPS systems take no time at all to transfer between utility power and battery power—0 milliseconds. Line-Interactive UPS systems, on the other hand, provide a slight delay during the transfer between utility power and battery power—4 milliseconds. While the vast majority of equipment can tolerate such a transfer, On-Line UPS systems set the highest standard.